1. Background of the Invention
The present invention relates to a seal which is arranged in an annular space, in which two members face each other, the two members are provided concentrically inside and outside of the space in diameter direction and which partitions the annular space into two spaces, i.e., a low pressure space and a high pressure space having a higher pressure than that of the low pressure space in axial direction, and also relates to a rotational assembly using the seal.
2. Description of Related Art
A seal which partitions a space into two spaces and seals one space from the other space is provided as a lip having an elastic element coated on an annular core metal and sliding the inner peripheral portion of the elastic element on a rotary member. A clearance between the lip and the rotary member is made a sealed portion by the seal and the leakage of oil from the clearance is thereby prevented. If such a seal is used to partition a space into a vacuum space and an atmospheric pressure space, the sealing performance of the seal is disadvantageously uneven despite the fact that the shape of the lip is the same.
After having been devoted to the intensive studies of the causes of unevenness in sealing performance, the inventors of the present invention made the following discoveries. If the axial contact width between the lip and the rotary member is larger and the sliding pressure of the lip with respect to the rotary member is higher, the sealing performance of the above-stated sealed portion is more improved.
On the other hand, if the axial contact width is larger and the sliding pressure is higher, the sliding resistance of the lip with respect to the rotary member is higher, which is unfavorable for the rotation characteristics of the rotary member. In view of this, the lip shape of a conventional seal is set such that the cross-sectional shape of the upper half of the lip is, for example, an inverse triangle having sharp edges, so as to make the axial contact width of the lip with respect to the rotary member as small as possible and to increase the sliding pressure of the lip as high as possible to thereby improve the sealing performance of the seal and so as to reduce the overall sliding resistance of the lip to thereby improve the rotation characteristics of the rotary member.
However, in case of the lip shaped as stated above, the range of the axial contact width of the lip is not specified. Due to this, it is impossible to make both the axial contact width and the sliding pressure constant because of the difference in production accuracy or in the material of the seal and it is difficult to manage the sealing performance of the seal while reducing the sliding resistance of the lip. As a result, the sealing performance becomes disadvantageously uneven.
As the above-stated seal, an oil seal has been conventionally used. In case of the oil seal, the material of the lip is ordinary rubber such as nitrile rubber, acrylic rubber or silicon rubber. Under the above-stated specific environment, therefore, inside gas or the like easily permeates into the lip itself due to the large difference in pressure between a sealing target space and an atmospheric pressure space, thereby disadvantageously deteriorating the sealing performance of the seal.
It is, therefore, an object of the present invention to provide a seal capable of managing the axial contact width between the a lip and one member to fall within a predetermined range to thereby manage sliding resistance to fall within a predetermined range and also capable of securing required sealing performance.
It is another object of the present invention to provide a seal capable of preventing gas from permeating into a lip itself under the specific environment as stated above and providing more improved sealing performance with respect to a sealing target space.
The other objects, features, advantages and the like of the present invention will be readily apparent from the following description.
A seal according to the present invention is constituted so that the axial contact width of a lip with respect to a rotary member falls within a range equal to or larger than a width necessary to secure minimum sealing performance with respect to the sealed portion and equal to or smaller than a width with which sliding resistance against the rotary member becomes a permissible upper limit, to thereby prevent the axial contact width from influencing rotation characteristics of the rotary member and to allow securing required sealing performance.
According to the present invention, if partitioning a space into a low pressure space having pressure of 10xe2x88x924 Pa or lower and an atmospheric pressure space, the axial contact width W0 of the lip with respect to the rotary member is specified to fall within the range of 0.2 mmxe2x89xa6W0xe2x89xa62.0 mm, to thereby prevent the axial contact width from influencing the rotation characteristics of the rotary member from more practical viewpoints and to allow securing the required sealing performance.
According to the present invention, the cross-sectional shape of the upper half of the lip is preferably a generally triangular shape directed toward a rotary member side, having the axial contact width gradually reduced and having an inside diameter-side vertex portion formed to be round. In that case, it is possible to ensure changing the axial contact width of the lip with respect to the rotary member to an axial contact width within the above-stated range when the lip slides on and contacts with the rotary member.
According to the present invention, the cross-sectional shape of the upper half of the lip is preferably a generally triangular shape directed toward the rotary member side and having the axial contact width gradually reduced and the lip is preferably bent from a root side to an inside diameter side. By doing so, if the slant face of the low pressure space-side lip contacts with the rotary member and the lip slides on and contacts with the rotary member, it is possible to ensure changing the axial contact width of the lip with respect to the rotary member to an axial contact width within the above-stated range.
According to the present invention, the interference of the lip with respect to the rotary member is preferably managed to hold the above-stated axial contact width. If so, it is possible to eliminate the influence on the rotation characteristics and to secure sealing performance more surely.
According to the present invention, the fastening force of the lip with respect to the rotary member is preferably managed to hold the above-stated axial contact width. If so, it is possible to eliminate the influence on the rotation characteristics and to secure sealing performance more surely.
According to the present invention a material having nitrogen gas permeability equal to or lower than a predetermined value is preferably selected as the material of the lip. If so, it is possible to prevent gas from penetrating into the lip itself toward the sealing target space side which is the low pressure space.
According to the present invention, the lip is preferably constituted out of a material having nitrogen gas permeability equal to or lower than 3.225 (cm3/m2/25 xcexcm/24 h/25xc2x0 C. atm). In that case, the seal can exhibit sealing performance even if the pressure of the low pressure space is equal to or lower than 10xe2x88x924 Pa and the seal can maintain the low pressure space having pressure of 10xe2x88x924 Pa or lower.